Statistical properties of LQC, QGC, and discoverable known lenses. First panel: distribution of the colour similarity, S_colour (we note that the vertical marker line for SDSS J1004+4112 falls on top of the line for SDSS J1029+2623, which makes the line for SDSS J1029+2623 difficult to see). Second panel: distribution of the Pan-STARRS1 g band magnitudes for all quasar images contained in each quasar candidate group or known lensed quasar system. Third panel: distribution of the maximum image separation. The blue and green curves are the simulation results derived from the model in Appendix B, corresponding to different lens mass profile models. Fourth panel: distribution of the photometric redshift z_ph for all quasar images in each group or known system; the blue curve shows the z_ph distribution for those quasar candidate groups in the QGC that have sufficient spectroscopic matches. Dashed lines indicate the positions of the corresponding values for the four discoverable known lenses in CatNorth. Stepped black histograms trace the distributions for the QGC.